Nuclear power reactors are a well known source of energy. In one type of nuclear reactor the nuclear fuel is comprised of elongated rods formed of sealed cladding tubes of suitable material, such as a zirconium alloy, containing uranium oxide and/or plutonium oxide as the nuclear fuel. A number of these fuel rods are grouped together and contained in an open-ended tubular flow channel to form a separately removable fuel assembly or bundle. A sufficient number of these fuel bundles are arranged in a matrix, approximating a right circular cylinder, to form the nuclear reactor core capable of self-sustaining a fission reaction. The core is submerged in a fluid, such as light water, which serves both as a coolant and as a neutron moderator.
A typical fuel bundle is formed by an array of spaced fuel rods supported between upper and lower tie plates; the rods typically being in excess of ten feet in length, on the order of one-half inch in diameter and spaced from one another by a fraction of an inch. To provide proper coolant flow past the fuel rods it is important to maintain the rods in precisely controlled, spaced relation such as to prevent bowing and vibration during reactor operation. A plurality of fuel rod spacers are thus utilized at spaced intervals along the length of the fuel bundle for this purpose.
Design considerations of such fuel rod bundle spacers include the following: retention of rod-to-rod spacing, retention of fuel bundle shape, allowance for fuel rod thermal expansion, restriction of fuel rod vibration, ease of fuel bundle assembly, minimization of contact areas between spacer and fuel rods, maintenance of structural integrity of the spacer under normal and abnormal (such as seismic) loads, minimization of reactor coolant flow distortion and restriction, maximization of thermal limits, minimization of parasitic neutron absorption, and minimization of manufacturing costs including adaptation to automated production.
Commonly assigned Matzner et al. U.S. Pat. No. 4,508,679 discloses and claims a nuclear fuel rod bundle spacer uniquely constructed to address these design concerns. As disclosed therein, a spacer is formed of an array of conjoined tubular cells or ferrules surrounded by a peripheral support band, each ferrule bore thus providing a passage through which a fuel rod or other elongated element of the fuel bundle is inserted. The ferrules are spot welded together and to the peripheral support band to provide an assembly of high structural strength.
The rods or elements extending through the ferrules are centered and laterally supported therein between rigid projections or stops and resilient members. The rigid projections or stops are inwardly formed as fluted or dimpled portions of the ferrule wall at locations near the upper and lower ferrule edges to maximize the axial distance therebetween and thus enhance fuel rod support.
The resilient members take the form of slender continuous loop springs of generally elliptical shape held captive by oppositely directed tabs formed by C-shaped cutouts in the walls of a pair of adjacent ferrules, whereby the two sides of each spring member project into the bores of its ferrule pair. Thus, a single spring serves two ferrules in biasing the fuel rods into contact with the two axially spaced pairs of stops pursuant to centering the rods in the ferrule bores.
In commonly assigned U.S. Patent Application entitled "Nuclear Fuel Bundle Spacer Spring Force Gauge", Ser. No. 07/659,664, filed Feb. 25, 1991, a gauge is disclosed for measuring the fuel rod-centering forces exerted by the two sides of the Matzner et al. double-acting springs assembled in a manufactured spacer as a quality assurance check prior to being put into nuclear reactor service. It is also desirable to determine the integrity of these springs at times of reactor servicing and/or refueling. It has been determined that the most reliable integrity check to determine whether long periods of high temperature and radiation exposure have degraded the utility of these springs is to measure their spring constant (ratio of spring force to spring deflection).